Technological advances in molecular biology, recombinant protein production and large scale protein purification have allowed the production of large amounts of proteins now used as biopharmaceuticals. For example, monoclonal antibodies and soluble forms of the TNF-a receptor have been used in the treatment of autoimmune diseases such as Crohn's disease or severe forms of psoriasis (1). Another example of use of recombinant protein is enzyme replacement therapy (ERT). ERT has been used to treat lysosomal storage diseases. This group of genetic disorders is characterized by the loss of function of lysosome enzymes resulting in severe somatic, and sometimes neuronal, pathologies. In ERT for these diseases, the patients are infused with large doses of normal enzymes. These infused enzymes are then internalized from circulation via cell surface receptors (mannose-6 phosphate receptor) and enter the endocytic pathway on their way to their site of action, the lysosome. Not all attempts to treat genetic disorders through ERT have been successful.
Hypophosphatasia is a rare, heritable type of rickets or osteomalacia that occurs with an incidence of 1 per 100,000 births for the more severe form of the disease. Milder forms are more prevalent. In this inborn metabolism defect, mutations inactivate the gene that encodes the tissue-nonspecific isoenzyme of alkaline phosphatase. It is characterized biochemically by subnormal alkaline phosphatase activity in serum. Alkaline phosphatase deficiency in osteoblasts and chondrocytes impairs skeletal mineralization, leading to rickets or osteomalacia.
There is a very broad range of expressivity of hypophosphatasia, spanning from a perinatal form often causing stillbirth from an unmineralized skeleton, to a milder form featuring only premature loss of teeth. Severely affected infants and children inherit hypophosphatasia as an autosomal recessive trait. There are four main forms of the disease: perinatal, infantile, childhood and adult. Perinatal hypophosphatasia manifests during gestation and most affected newborns survive only briefly. Infantile hypophosphatasia becomes clinically apparent before 6 months of age. About 50% of patients die within a year. Childhood hypophosphatasia varies greatly in severity but most of these patients will suffer from skeletal symptoms throughout their life. Adult hypophosphatasia appears during middle age, with symptoms such as painful recurrent stress fractures having poor healing.
Osteoblasts and chondrocytes are normally rich in tissue-nonspecific alkaline phosphatase where it is attached to the cell surface. In hypophosphatasia, the lack of alkaline phosphatase activity results in the extracellular accumulation of three phosphorus-compounds believed to be substrates of the enzyme: phosphoethanolamine (PEA), inorganic pyrophosphate (PPi) and pyridoxal 5′-phosphate (PLP). PPi is an inhibitor of hydroxyapatite crystal growth, and PPi build-up in the disease accounts for the impaired skeletal mineralization. Consequently, providing active enzyme to patients suffering from hypophosphatasia will decrease extracellular PPi levels and improve skeletal mineralization.
Currently, there is no established medical therapy for hypophosphatasia. Trials of enzyme replacement using intravenous infusions of alkaline phosphatase have failed. It appears that alkaline phosphatase activity must be increased not in circulation but in the skeleton itself. This hypothesis was confirmed recently by bone marrow transplantation. Unfortunately, the benefits of the transplantation lasted only for a short period of time due to poor engraftment.
There is a therefore a need to provide enzyme replacement therapy approach to provide active enzyme to the skeleton of patients suffering from hypophosphatasia.
Bone-targeted proteins could be useful not only for the treatment or prevention of hypophosphatasia (loss of function of alkaline phosphatase) but also for the treatment or prevention of other genetic diseases characterized by defective enzymatic activity involved in bone metabolism, such as X-linked hypophosphatemic rickets (XLH) (loss of function of phosphate regulating gene with homology to endopeptidases on the X chromosome (PHEX)).
XLH is the most prevalent of the familial hypophosphatemias (OMIM 307800, 307810). It is characterized by reduced phosphate reuptake in the kidney, hypophosphatemia, normocalcemia, normal to low plasma 1,25-dihydroxyvitamin D3 (1,25(OH)2D, calcitriol) levels, normal parathyroid gland function and elevated plasma alkaline phosphatase activity. These changes are associated with growth retardation, lower extremity deformity, radiologic and histomorphometric evidence of rickets and osteomalacia. This disease appears to result from combined renal defects in tubular phosphate reabsorption and vitamin D metabolism, as well as a functional disorder in bone and teeth. XLH results from inactivating mutations in the PHEX gene, a member of the zinc metallopeptidase family of type II integral membrane glycoproteins. These mutations prevent the expression of a functional PHEX enzyme at the cell surface of osteoblasts. As of now, treatment of XLH patients is restricted to supplementation with oral inorganic phosphate (Pi) supplements in four or five divided doses per day, and co-administration of 1,25(OH)2D to compensate for the inappropriate synthesis of 1,25(OH)2D. Such high doses of phosphate frequently cause gastrointestinal intolerances, particularly diarrhea, leading to patient non-compliance. On the one hand, the phosphate load carries the risk of provoking secondary hyperparathyroidism (which may be severe enough to necessitate parathyroidectomy) while on the other hand, administration of excess 1,25(OH)2D may lead to hypercalciuria, hypercalcemia and nephrocalcinosis.
Useful ERT for XLH would therefore seek to replace the defective PHEX enzyme in XLH patients with a functional enzyme obtained through recombinant DNA technology. As the normal PHEX enzyme is anchored in osteoblast plasma membrane by a hydrophobic peptide, the natural form of PHEX cannot be produced and purified in sufficient quantities to be used in a pharmaceutical preparation. To circumvent the problem, a soluble form of recombinant PHEX (or sPHEX) was engineered and produced in cell cultures, purified and formulated for intravenous (IV) administration (WO 00/50580). sPHEX was then injected in Hyp mice, a mouse model for XLH, as described in co-pending U.S. application Ser. No. 10/362,259. Improvement of several bone related serum parameter were observed including a reduction of the abnormally high levels of serum alkaline phosphatase. Although these experiments were successful, it was believed that the efficacy of therapeutic sPHEX might be enhanced if the recombinant protein was modified so as to promote its binding to bone minerals.
There is therefore a need for means to successfully target proteins to bone matrix.
Biphosphonates are known to present high affinity binding to hydroxyapatite (HA), and has been used to target small molecules (4) and proteins (5) to bones. However this strategy requires chemical modifications of the purified proteins, and presents several disadvantages including possible interference with protein activity and additional purification steps.
Another strategy to target small molecules to bone has been to conjugate these entities to acidic peptides such as poly-Asp (6). This strategy was developed after the observation that several proteins synthesized by osteoblasts, the bone forming cells, bind to bone matrix through sequences particularly rich in acidic amino acid residues (Asp and Glu). This is the case of osteopontin (7) and bone sialoprotein, two noncollagenous proteins. Hence acidic peptides (E2-10 and D2-10) were used to target small molecules (i.e. methotrexate, FITC, Fmoc, biotin, estradiol) to hydroxyapatite in vitro. Acidic peptides (E6 and D6-10) were used to target small molecules (i.e. FITC, Fmoc, estradiol) to hydroxyapatite in vivo. Finally, E6 was shown to confer to BSA, hemoglobin and IgG the ability to bind hydroxyapatite in vitro. In all the above cases, linking of the acidic sequence was performed chemically.
The present invention seeks to meet these needs and other needs.
The present description refers to a number of documents, the content of which is herein incorporated by reference in their entirety.